As is well known, internal combustion engines work by mixing fuel with air and combusting the mixture in one or more engine cylinders to reciprocate pistons in the cylinders, with the pistons in turn being coupled to the crankshaft of the engine to turn the crankshaft and, thus, to output power. As is also well known, to increase engine efficiency and to prolong engine life, dust, particles, and other particulate should be filtered out of the air, prior to combustion. Accordingly, most vehicles are equipped with the familiar vehicle engine air filter, which includes a hollow housing that holds a filter element. Air from the engine compartment is directed through an air inlet plenum into the housing, inwardly through the filter element, and out of an outlet plenum. The filtered air is then mixed with gasoline either in a carburetor or in the cylinders of the engine.
Not surprisingly, many air filters for vehicle engines have been introduced. For example, U.S. Pat. No. 4,211,543 discloses an elongated replaceable paper filter element that fits into a urethane outer liner. An inner liner is nested in a safety sleeve, and the safety sleeve in turn is received within the filter element. The filter disclosed in the '543 patent is intended primarily for truck engines, however, and is not necessarily optimized for the engines of other types of vehicles. Specifically, the present invention recognizes that owing to its evident lack of non-filtering structure external to the filtering media, the '543 patent does not facilitate the directing of air into the filter beyond simply allowing the air to enter the filter. Also, the internal structure of the '543 patent does not appear to be designed with the purpose of effectively redirecting and smoothing air flow within the filter.
U.S. Pat. No. 3,385,039, on the other hand, discloses an air filter for non-vehicular ventilation systems which includes an outer grid shaped like a rectangular pyramid, with the upper, smaller portion of the pyramidal grid being inverted with respect to the lower, larger portion. A complementarily-configured inner grid nests within the outer grid, and a filter medium is held between the grids. The '039 patent, however, is not intended for vehicle use, and consequently the '039 patent envisions air flow proceeding from inside the grid to outside the grid, not vice-versa. Perhaps not surprisingly, therefore, the '039 structure is not optimized for redirecting and smoothing air flow within its grids, nor is it optimized for directing air flow outside its grids to flow inwardly, since the air flow in the '039 patent is outward. As recognized by the present invention, in a vehicle air filter in which air flows inwardly to the center of the filter, it is desirable to smooth and redirect air within the filter as the air passes to the outlet plenum of the filter.
Additionally, the '039 filter, while seeking to optimize air filtration surface area by its structure, uses two (an inner inverted set and an outer set) sets of four flat walls each, owing to the generally pyramidal structure of the filter. As recognized herein, however, it is possible to further increase the air filtration surface of a vehicle air filter beyond what is envisioned in the '039 patent. Moreover, the '039 filter requires locks to hold the grids together, and a seal to hold the filter medium between the grids, undesirably complicating the structure of the '039 filter.
Other structural shortcomings exist in prior art filters. For example, we understand that polyurethane foam filters are effective, but existing filters that use polyurethane as the filtering medium use highly restrictive metal meshes to keep their foams securely in place. This impedes air flow into the filters and thus is undesirable.
In addition to the above considerations, we have discovered that engine performance at higher speeds and power is better optimized using air filters having relatively high porosity and, thus, relatively low resistances to air flow, whereas engine performance at lower speeds is better optimized using air filters having relatively low porosity and, thus, relatively high resistances to air flow. We think this is because the resonance of the air intake system of an engine is better tuned to the resonance of the engine RPM (thereby increasing air pressure at the cylinder and thus boosting power at the target RPM) by establishing air filter resistances as summarized above. Unfortunately, standard vehicle engine air filters typically do not envision or provide for selecting more than a single, "one size fits all" filter element type for any particular vehicle model.
Accordingly, it is an object of the present invention to provide an improved vehicle engine air filter.